1. Field of the Invention
The invention relates to magneto-optical recording media and apparatus for the readout of magneto-optically recorded data on the magneto-optical recording media.
2. Description of the Related Art
Currently, when information is recorded by a magneto-optical disc apparatus, a laser beam of light is typically focused by a lens on the disc as a spot having a wavelength at the order of the diffraction limit, while modulating the intensity of the laser beam by a code data signal to be recorded, for example. As a result, the temperature of the magnetic recording media such as the RE-TM amorphous thin film alloy, which is an alloy of amorphous rare earth metal and a transition metal such as Te-Fe-Co or Gd-Tb-Fe, is increased to 150-200 degrees centigrade. As the recording medium is irradiated and heated by the laser, the temperature of the media increases beyond the Curie temperature (Tc), and the coercivity of the media is lost. By applying a magnetic bias field in a certain direction by a magnet, a magnetization reversal occurs and a magnetic domain is recorded after the heated portion returns to room temperature.
On the other hand, the readout of the code data thus recorded on the disc is performed by focusing a laser beam of light having a certain output power on the disc. As the focused spot of light is reflected by the magneto-optical recording media surface, the polarization of the laser beam is varied by the Kerr effect, so that by detecting optically the variation in polarization of the reflected light, information of the magnetization recorded on the disc can be read.
The aforementioned techniques have been utilized in the 3.5 or 5.25 inch magneto-optical disc drives according to the ISO standard. The diameter D of the focused optical spot can be theoretically expressed by an equation D=0.5.lambda./NA, where .lambda. is the wavelength of the laser and NA is the numerical aperture of the lens. If a semiconductor laser with the wavelength .lambda. of 0.68 .mu.m and the lens numerical aperture NA of 0.55 is used, for example, the optical spot diameter D would be 0.6 .mu.m, and readout of magnetic data smaller than this diameter would be very difficult.
For example, JP Pat. Appln. Disclosure No. 3-93056 and ajournal "Magnetically Induced Super Resolution Optical Disk" of the Magnetic Society of Japan, 15-5, pp. 838-844 (1991), disclose a method of reading out magnetic data d smaller than the optical spot diameter D, by which method a rotating disc is scanned by a laser beam of light with slightly larger power than for the ordinary readout in order to take advantage of the temperature distribution in the magnetic media within the irradiated light spot region on the disc surface. This technique is a magnetically induced super resolution technique called FAD (Front Aperture Detection) method. The FAD method, however, has disadvantages such as requiring a large DC magnetic field during readout, and therefore has not been established as a practical method of data readout.
The problems of the prior art will be explained by referring to this magneto-optical disc recording system utilizing the laser pulse irradiation and magnetic field modulation, and the method of reproducing such recording as shown in FIGS. 3 and 4.
In FIG. 5(a), a laser diode 1 is driven by a laser driver 9 in accordance with a clock signal 10 reproduced and generated from a phase pit mark on the magneto-optical recording disc 8. The resultant pulsed light (whose waveform is indicated at 2) is irradiated onto the magneto-optical recording medium 8 as an optical spot 4 by an objective lens 3. A modulated magnetic field 7 is formed by a data signal generator 6 using a magnetic head 5 arranged near the disc. The optical spot 4 is focused and irradiated onto the disc surface and the pulsed laser beam 2 in synchronization with the clock 10 and the modulated magnetic field 7 are combined, whereby irradiation spots 4 are written in an overlapping manner and a crescent shaped magnetic domain 11 which has a smaller mark length than the optical spot diameter D can be recorded. This method is known from JP Pat. Appln. Disclosure No. 1-292603. When the laser wavelength .lambda. is 0.68 .mu.m and the lens numerical aperture NA is 0.55, the focused optical spot diameter D will be 0.62 .mu.m. By reducing the pulse irradiating interval, a crescent shaped magnetic domain with a short mark length d=0.1 to 0.2 .mu.m and the mark width substantially equal to the optical spot diameter D can be recorded. In a disc with the diameter of 120 mm, if the track pitch p is set to 0.6 .mu.m and the shortest mark length d to 0.26 .mu.m, seven to ten GB (Giga bytes) capacity of information can be recorded. However, such magnetic data with the shortest mark length d=0.1 to 0.2 .mu.m is extremely difficult to reproduce with a focused spot with a diameter D=0.62 .mu.m (D=0.5 .lambda./NA).
FIG. 4 is for the explanation of the FAD (Front Aperture Detection) system whereby magnetic pits smaller than the optical spot 4 can be read using an MSR (Magnetically Induced Super Resolution) method. This technique is known from JP Pat. Appln. Disclosure No. 3-930567 or the already-mentioned journal of the Magnetics Society of Japan, 15-5, pp. 838-844 (1991) "Magnetically Induced Super Resolution Optical Disk." In FIG. 4(a), an opto-magnetic medium 12 comprises three layers having different magnetic and temperature characteristics, i.e., a memory layer 12-1, a read-out layer 12-2, and a switching layer 12-3. As shown in FIG. 4(b), as an optical spot 4 scans a track on the magnetic media surface 12 where magnetic data 15, 15-1 are recorded, the optical energy is absorbed by the magnetic recording medium and there results a temperature distribution in the magnetic recording medium within the spot. As a result, in a high temperature region 13-1 of the switching layer 12-3 where the temperature is above the Curie temperature Tc=140 degrees centigrade, the coercivity of the switching layer disappears and thus the exchange-coupling between the memory layer and read-out layer is lost. Upon the application of a readout magnetic field Hr, the magnetization directions of the read-out layer 12-2 with a smaller coercivity are aligned by the readout magnetic field Hr. Accordingly, the magnetized pit 15-1 in the high temperature region 13-1 on the memory layer where the optical spot is irradiated is masked, so that only the magnetized pit 13 in the low temperature region 13-2 can be transferred and read out. This system, however, reads the chevron-shaped low temperature region, and although a career to noise (C/N) level of 45 dB can be achieved with respect to the 0.4 .mu.m mark length, as the mark length becomes 0.3 .mu.m, for example, the career to noise level decreases down to 30 dB. Furthermore, since the mask shape in the reproduced region becomes an inversed-chevron shape as mentioned above, the disadvantage arises that as the track pitch is reduced, a leakage of reproduced signal from a neighboring track becomes significant and therefore the track density cannot be improved.
Referring now to FIG. 3, the DWDD (Domain Wall Displacement Detection) system as known from ajournal of the MORIS/ISOM Society "High Density Magneto-Optical Recording with Domain Wall Displacement Detection," Tu-E-0438-38(1997) and JP Pat. Appln. Disclosure No. 6-290496 will be explained.
FIG. 3(a) shows the structure of the magnetic medium 12 according to the DWDD system, which comprises a memory layer 12-1, a displacement layer 12-2-2, and a switching layer 12-3. In the DWDD system, as the laser is irradiated, a temperature gradient is formed along the movement of the spot. As a result, as the temperature of the magnetic layer of the switching layer exceeds the Curie temperature, the switching layer loses its coercivity and the exchange coupling between the memory layer 12-1 and the displacement layer 12-2-1 is weakened, which causes the domain wall formed in the high temperature region in the displacement layer to move along the temperature gradient, and the recorded information can be reproduced by detecting the changes in the polarization of the reflected optical beam. More specifically, in FIG. 3(a), as the track in which magnetic information is recorded is scanned by a DC irradiation of the laser, the optical energy is absorbed by the magnetic medium and there appears a temperature distribution in the magnetic medium within the spot 4 as shown in FIG. 3(b). Consequently, in the high temperature region 13-1 (as indicated by the hatching), where the temperature is above the Curie temperature Tc=140 degrees centigrade of the switching layer, the exchange-coupling between the memory layer and read-out layer is lost. On the other hand, in a region of the switching layer where the temperature is below the Tc, the memory layer exhibits a large frictional force due to the exchange-coupling.
Referring to FIG. 3(b), the gradient of the temperature distribution in the displacement layer theoretically corresponds, as explained in JP Pat. Appln. Disclosure No. 6-290496, to the force by which the domain wall is displaced. Accordingly upon the laser irradiation the magnetic wall formed in the high temperature region of the displacement layer is drawn in a direction of larger temperature gradient, and as a result the minute magnetic domain 15 in the memory layer is extended in the displacement layer, thereby allowing the magnetic pits in the memory layer to be read as a large readout output signal by the extension of the displacement layer. The DWDD Method can reproduce smaller minute magnetic domain than can the FAD method; however, this method utilizes a temperature gradient which is less steep, as shown in FIG. 3(b), in the high temperature region, the force by which the magnetic wall in the displacement layer is moved is weak and thus the domain wall displacement cannot go smoothly. Further, as discussed in "High Density Magneto-Optical Recording with Domain Wall Displacement Detection" by the MORIS/ISOM Society, Tu-E-0438-38 (1997), this laser DC readout system, in which the temperature gradient is gentle, results in a slower velocity in the movement of the domain wall, thereby causing a ghost image to appear in the reproduced waveform and preventing an accurate readout of the data in the memory layer. In order to eliminate this ghost image, a large read out DC magnetic field of the order of several hundred oersted is required for readout and this posed a problem similar to the aforementioned FAD method.